1. Field of the Invention
The present invention refers to fluid pumps.
2. Description of Prior Art
It is known to use positive-displacement pumps for transporting liquids and gases, said positive-displacement pumps consisting of a periodic displacer, a piston or a diaphragm, and two passive check valves. Due to the periodic movement of the piston or of the diaphragm, liquid is drawn into a pump chamber through the inlet valve and displaced from said pump chamber through the outlet valve. The direction of transport is predetermined by the arrangement of the valves. When the pumping direction of such an arrangement is to be reversed, such known pumps require a change of the operating direction of the valves from outside which entails a high expenditure. Such pumps are shown e.g. in Jarolav and Monika Ivantysyn; "Hydrostatische Pumpen and Motoren"; Vogel Buchverlag, Wurzburg, 1993.
Pumps of this type having a small constructional size and delivering small pumped streams are referred to as micropumps. The displacers of such pumps are typically implemented as a diaphragm, cf. P. Gravesen, J. Branebjerg, O. S. Jensen; Microfluids--A review; Micro Mechanics Europe Neuchatel, 1993, pages 143-164. The displacers can be driven by different mechanisms. Piezoelectric drive mechanisms are shown in H. T. G. Van Lintel, F. C. M. Van de Pol. S. Bouwstra, A Piezoelectric Micropump Based on Micromachining of Silicon, Sensors & Actuators, 15, pages 153-167, 1988, S. Shoji, S. Nakagawa and M. Esashi, Micropump and sample injector for integrated chemical analyzing systems; Sensors and Actuators, A21-A23 (1990), pages 189-192, E. Stemme, G. Stemme; A valveless diffuser/nozzle based fluid pump; Sensors & Actuators A, 39 (1993) 159-167, and T. Gerlach, H. Wurmus; Working principle and performance of the dynamic micropump; Proc. MEMS'95; (1995), pages 221-226; Amsterdam, The Netherlands. Thermopneumatic mechanisms for driving the displacers are shown in F. C. M. Van de Pol, H. T. G. Van Lintel, M. Elwenspoek and J. H. J. Fluitman, A Termo-pneumatic Micropump Based on Micro-engineering Techniques, Sensors & Actuators, A21-A23, pages 198-202, 1990, B. Bustgens, W. Bacher, W. Menz, W. K. Schomburg; Micropump manufactured by thermoplastic molding; Proc. MEMS'94; (1994), pages 18-21. An electrostatic mechanism is shown in R. Zengerle, W. Geiger, M. Richter, J. Ulrich, S. Kluge, A. Richter; Application of Micro Diaphragm Pumps in Microfluid Systems; Proc. Actuator '94; 15.-17.6.1994; Bremen, Germany; pages 25-29. Furthermore, the displacers can be driven thermomechanically or magnetically.
As is also shown in the above-mentioned publications, either passive check valves or special flow nozzles can be used as valves. The direction of transport of micropumps can be reversed without forcibly controlling the valves, simply by effecting control at a frequency above the resonant frequency of said valves. In this context R. Zengerle, S. Kluge, M. Richter, A. Richter; A Bidirectional Silicon Micropump; Proc. MEMS '95; Amsterdam, Netherlands; pages 19-24, J. Ulrich, H. Fuller, R. Zengerle; Static and dynamic flow simulation through a KOH-etched micro valve; Proc. TRANSDUCERS '95, Stockholm, Sweden, (1995), pages 17-20, should be taken into account. The cause of this effect is a phase displacement between the movement of the displacer and the opening state of the valves. If the phase difference exceeds 90.degree., the opening state of the valves is anticyclic to their state in the normal forward mode and the pumping direction is reversed. A change of the operating direction of the valves from outside of the type required when macroscopic pumps are used can be dispensed with. The decisive phase difference between the displacer and the valves depends on the drive frequency of the pump on the one hand and on the resonant frequency of the movable valve member in the liquid surroundings on the other.
One disadvantage of this embodiment is to be seen in the fact that, upon constructing the valves, a compromise has to be found between the mechanical resonance in the liquid surroundings, the flow resistance, the fluidic capacity, i.e. the elastic volume deformation, the constructional size and the mechanical stability of these valves. It follows that these parameters, each of which may influence the pumping dynamics, cannot be ajusted to an optimum value independently of one another and part of them is opposed to a desired further miniaturization of the pump dimensions.
A general disadvantage entailed by the use of pumps with passive check valves is also the fact that, when switched off, the pumps do not block the medium to be transported. If the input pressure exceeds the output pressure by the pretension of the valves, the medium to be pumped will flow through the pump.
Micropumps using special flow nozzles have the disadvantage that they have a very low maximum pumping efficiency in the range of 10 to 20%.
A micropump of the type discussed, which is provided with check valves, is disclosed e.g. in EP 0 568 902 A2. This micropump is driven by means of the reciprocal movement of a diaphragm. The movement of the diaphragm causes a change in the volume of a pump chamber defined by the diaphragm and a carrier component. The outlet and the inlet of the micropump are provided with an outlet valve and an inlet valve, respectively.
WO-A-87/07218 discloses a piezoelectrically driven pressure-generating means comprising an electrically controllable diaphragm consisting of a first piezoelectrically excitable layer and a support layer which is fixedly connected to said excitable layer. The diaphram has a piezoelectrically excitable peripheral area and a piezoelectrically excitable central area, said areas being controlled in such a way that, for causing diaphram deflection, the diaphragm is reduced in length in its peripheral deflection, the diamphragm is reduced in length in its peripheral area by transverse contraction and increased in length in its central area. WO-A-87/07218 additionally discloses a fluid pump which makes use of three interconnected diaphragms of the type described hereinbefore, a first diaphragm serving as an inlet valve, a second diaphragm delimiting a variable hollow space and a third diaphragm serving as an outlet valve.
FR-A-2478220 discloses a pump in the case of which two drive means are provided for moving a flexible diaphragm, which is provided with a movable plate, into different end positions. The diaphragm is attached to a carrier plate having a central inlet opening. The diaphragm is provided with outlet openings. A pumping effect from the inlet opening to the outlet openings can be produced by controlling the diaphragm in a suitable manner.